Hitherto, plasma torches have been provided with a nozzle around an electrode to form a path for an operating gas, and have generated a hot plasma jet between the electrode and the cutting material to provide a good cut quality. One of the most effective ways of increasing the output of the plasma jet is to increase the amount of discharge current by increasing the cooling effect. Increasing the size of the gap between the electrode and the nozzle in the plasma torch is another way of achieving this end. Increasing the gap size, however, produces a long arc column between the electrode and the nozzle, so that the position where the arc is generated at the nozzle side tends to vary in an irregular fashion. This often results in an unstable output. A technique which overcomes such a problem is the swirling operating gas technique. The formed swirl has a center region which is lower in pressure than its peripheral region, so that even when the aforementioned long arc column is generated, this arc column gets trapped in the aforementioned low pressure region, which prevents variations from occurring in the position where the arc is generated at the nozzle side. As a result of this, a stable output can be produced.
A known construction, of a plasma torch which forms this swirl, has a swirler disposed along the outer periphery of the electrode and a plurality of jet ports which are formed obliquely from the outer periphery to the inner periphery of the swirler, whereby the plasma torch ejects operating gas from the jet ports in a direction tangent to an inner peripheral circle of the swirler. In such a construction, however, the electrode axial component, in addition to the swirling direction component, is strongly present. Therefore, it is not easy to obtain an intense swirl, that is a swirl having a swirling component as the main component.
A plasma torch which ejects operating gas, with the aforementioned electrode axial direction component minimized, is shown in FIGS. 6 and 7. This plasma torch is constructed so that operating gas G is ejected in the horizontal direction. In other words, this plasma torch is so constructed as to have a swirler 4 disposed along the inner periphery of the upper portion of a nozzle 5; a swirler chamber 2 surrounded by an electrode 1, the swirler 4, and the nozzle 5; and a plurality of jet holes 3a formed in a direction tangent to an inner wall 4a of the swirler 4; whereby the plasma torch ejects the operating gas G from jet ports 3 of the jet holes to form a swirl S (refer to, for example, Japanese Patent Laid-Open Nos. 63-250097 and 3-174980). In the figures, reference numeral 4A denotes a transverse section of the swirler 4.
The above-described swirl technique, though an excellent technique to stabilize the plasma jet, does not provide enough stability, so that a large quantity of adhered dross (molten metal) during cutting is observed. Such plasma torches will be described with reference to FIG. 5. None of the plasma torches investigated by the inventor were found to have their jet ports arranged in accordance with a positional relationship (d/D) of d/D&lt;0.06, like the jet ports 3 of the various plasma torches with the above-described constructions. Here, D stands for a diameter of a swirler chamber (or inner diameter of a swirler 50) in each transverse section 2A of the swirler 50 including the jet ports 3. On the other hand, d stands for a minimum distance, in each transverse cross-section 2A of the aforementioned swirler 50, from a line 2b, which is tangent to the diameter D of the swirler chamber and parallel to a projected line 3c of an axis 3b of the jet hole 3a, to a side end 2c of the side of the jet port 3 adjacent to the tangential line 2b.